The present invention relates to fluorescent lamps. More particularly, it relates to fluorescent lamps having a phosphor coating that is substantially carbonate-free.
Fluorescent lamps utilizing a rare earth blended phosphor system are well known in the art. Conventional rare earth phosphor blends are disclosed, e.g., in U.S. Pat. Nos. 5,045,752, 4,088,923, 4,335,330, 4,847,533, 4,806,824, 3,937,998, and 4,431,941. Typically, a rare earth phosphor layer utilizing a conventional rare earth phosphor blend comprises a mixture of red, green and blue emitting rare earth phosphors resulting in a triphosphor mixture.
In the conventional triphosphor system, the particular red, green and blue emitting phosphors, as well as their weight percents, are selected based on their specific color emission characteristics so that when combined, their respective color emissions result in light output of the desired color with suitable color rendering properties (CRI) for the intended application.
The red emitting phosphor is most commonly a yttria-based species, e.g. yttrium oxide activated with europium (Eu3+), commonly abbreviated YEO. The green emitting phosphor can be lanthanum phosphate activated with cerium (Ce3+) and terbium (Tb3+), commonly abbreviated LAP, less preferably cerium, magnesium aluminate activated with terbium (Tb3+), commonly abbreviated CAT, still less preferably gadolinium, magnesium pentaborate activated with cerium (Ce3+) and terbium (Tb3+), commonly abbreviated CBT. The blue emitting phosphor is most commonly calcium-based, such as calcium, strontium, barium chloroapatite activated with europium (Eu2+). Less preferably, the blue phosphor can be barium, magnesium aluminate activated with europium (Eu2+). Alternatively, other red, green and blue emitting rare earth phosphors are known and could be selected based on operational as well as cost considerations.
In addition to the above-described triphosphor system, rare earth phosphor blends comprising other numbers of rare earth phosphors, such as systems with 4 or 5 rare earth phosphors, are also known and can be used. Still further, phosphor layers having a mixture of rare earth phosphors and the less expensive halophosphors can also be used.
In virtually all rare earth phosphor blends, as well as blends containing both rare earth and halophosphors, it is necessary or desirable to include a red emitting rare earth phosphor. As described above the most common red emitting rare earth phosphors are yttria-based. Though other red emitting rare earth phosphors are known, the yttria species of phosphors are highly preferred due to their excellent red color emitting characteristics, including color intensity and resulting CRI in the red spectrum.
The process for making a fluorescent lamp involves coating the interior surface of the lamp's glass envelope with a slurry of the desired phosphors. Then the slurry is dried or heated to decompose or vaporize liquid slurry components leaving behind the dried coating of phosphors.
The slurry is prepared as an aqueous slurry suspension of the phosphors (which are generally provided in powdered form). The aqueous slurry must have sufficient viscosity to effectively and uniformly coat the interior surface of the glass envelope prior to drying. Conventionally, acrylic thickeners containing carboxylic acid (COOH) groups are used to to elevate and regulate the slurry viscosity. However, when yttria-based rare earth phosphors are used, the slightly water-soluble yttria reacts with the COOH groups on the acrylic molecules causing a cross-linking cascade that effectively cross-links the acrylic molecules causing gellation or flocculation of the slurry. The result is that the slurry essentially “gels,” and is no longer in a liquid or flowable state.
To avoid the above complication, other polymeric thickeners having no COOH groups are used when yttria-based red phosphors are to be included in the phosphor slurry. In this case, the thickener of choice is polyethylene oxide (PEO). PEO is a highly effective thickener for aqueous systems, and has been used with great success to regulate the viscosity of phosphor slurries for coating onto the glass envelope of a fluorescent lamp. However, PEO conventionally contains trace amounts of carbonate salts, particularly calcium carbonate particles, as a result of the manufacturing process for PEO. Conventionally available PEO contains from 0.2 to about 1 percent calcium carbonate by weight.
The presence of this small amount of calcium carbonate in the PEO thickener has been well known to lamp manufactures, and till now has been believed to be innocuous to lamp production and/or performance. However, the inventor herein has discovered, surprisingly and unexpectedly, that the presence of this small amount of calcium carbonate in the PEO thickener results in calcium carbonate being present in the finished phosphor layer, and causes lower lumen efficiency and lumen maintenance in the finished fluorescent lamp. Accordingly, there is a need in the art for a fluorescent lamp that utilizes yttria-based red emitting rare earth phosphors, yet contains no calcium carbonate in the phosphor layer.